Most telephone service subscribers recognize that a ringing telephone indicates an incoming telephone call. In many cases, the telephone station rings in response to the receipt of a ring signal. For example, a telephone company providing Plain Old Telephony Service (POTS) may receive a call for a given subscriber and may initiate the sending of a ring signal to the home of that subscriber. A common technique involves feeding a 75 Volt, 20 Hz Alternating Current ringing current down one wire of a twisted pair telephone line. The signal is not often applied constantly to the line. The signal is usually applied in a pattern. For example, a United States service provider may use a repeated pattern that is two seconds on and four seconds off.
In some circumstances, a service provider may elect to use different ring signal patterns. For example, some service providers have begun to offer a distinctive ringing option to their subscribers. A subscriber may have multiple telephone numbers assigned to one physical telephone line. By assigning different signal patterns to each of the different telephone numbers, a service provider may be able to cause a subscriber telephone station to ring with a distinctive ring tone—indicating to the subscriber which of the telephone numbers is being called.